I am unable to use the Aeroplan site, but I think that you can select "Round Trip", "One Way" or "Multi-leg" defaults. Use the "Multi-leg" one and try booking by entering YOW-YYZ;YYZ-SFO;SFO-YYZ;YYZ-YOW all as separate legs. Remember to indicate "Executive Class" too. This should give you each segment you want. A bit more of a process, but should work. Otherwise you would be looking at the Service Charge by using an agent.